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Geothermal Energy

Geothermal energy is natural heat from the Earth's interior where temperatures reach greater than 7,000°F. The heat is brought to the surface as steam or hot water—created when water flows through heated, permeable rock—and used directly for space heating in homes and buildings or converted to electricity. The most rapidly growing use for geothermal energy is geothermal heat pumps, which use earth or low-temperature groundwater as a heat source in the winter and a heat sink in the summer.

The current production of geothermal energy from all uses places third among renewables, following biomass and hydroelectricity, and ahead of solar and wind. Yet this production has barely scratched the surface of the enormous potential of geothermal energy. U.S. geothermal resources alone are estimated at 70,000,000 quads, equivalent to a 750,000-year supply of energy for the entire nation at current rates of consumption. The geothermal energy potential in the uppermost 6 miles of the Earth's crust amounts to 50,000 times the energy of all oil and gas resources in the world. Visit the U.S. Department of Energy's Geothermal Technologies Program website to view Geothermal Resource Maps.

Although numerous types of geothermal resources are under development, only two resources can be used commercially with today's technologies: hydrothermal fluids and earth energy.

Hydrothermal fluid resources are tapped for electricity generation. These resources are reservoirs of steam or hot water that are formed by water seeping into the Earth, collecting in, and being heated by fractured or porous hot rock. These reservoirs are tapped by drilling wells to deliver hot water to the surface for generation of electricity or direct use. In the United States, the hottest (and currently most valuable) resources are in the western states, Alaska, and Hawaii.

Earth energy is the heat contained in soil and rocks at shallow depths. This form of low and moderate temperature resources is tapped for direct use and ground source heat pumps.

Because of the variety of applications for geothermal energy, the technologies for extracting the resource differs widely. The focus here is on power generation, direct use, and geothermal heat pumps.

Power Generation

Three technologies can be used to convert hydrothermal fluids to electricity. The type of conversion used depends on whether the fluid is steam or water, and its temperature.

• Steam: Conventional steam turbines are used with hydrothermal fluids that are wholly or primarily steam. The steam is routed directly to the turbine, which drives an electric generator, eliminating the need for the boilers and fossil fuel of conventional power plants.

• High-temperature water: For hydrothermal fluids hotter than 400°F that are primarily water, flash steam technology is usually employed. In these systems, the fluid is sprayed into a tank held at a much lower pressure than the fluid, causing some of the fluid to rapidly vaporize, or flash, to steam. The steam is used to drive a turbine, which again drives a generator.

• Moderate-temperature water: For water with temperatures lower than 400°F, binary cycle technology is generally most cost effective. In these systems, the hot geothermal fluid vaporizes a secondary—or working—fluid, which then drives a turbine and generator. Because lower temperature waters are much more plentiful than high-temperature waters, binary cycle systems will be the dominant geothermal power plants of the future.

Direct use: Direct-use projects generally use resource temperatures of 100°-300°F. These projects include heating buildings, industrial processes, greenhouses, fish farms, and resorts. Current installed U.S. capacity of direct-use systems totals 500 megawatts (MW)-thermal or enough to heat 40,000 average-sized homes.

In a typical direct-use application, a well brings heated water to the surface; a mechanical system—piping, heat exchanger, controls—delivers the heat to the space or process; and a disposal system either injects the cooled geothermal fluid underground or disposes of it on the surface.

Geothermal Heat Pumps

Ground-source heat pumps, also called geothermal heat pumps, use the Earth or groundwater as a heat source in winter and a heat sink in summer. The heat pump, a device that moves heat from one place to another, transfers heat from the soil to the house in winter and from the house to the soil in summer. About 500,000 ground-source heat pumps are being used for heating and cooling throughout the United States in residential, commercial, and government buildings, including more than 500 in schools. Visit geothermal heat pumps for more information.

Geothermal energy offers an environmentally benign source of electricity that is reliable and cost effective. Today's hydrothermal power plants with modern emissions controls have minimal impact on the environment. The plants release little or no carbon dioxide, a greenhouse gas suspected of contributing to global warming.

Geothermal power plants are very reliable compared to conventional power plants. Taken as a group, geothermal power plants can generate power 95% or more of the time; they are seldom off-line for maintenance or repair. Not only are they reliable, but their capacity factor—the ratio of the amount of electricity a plant produces to how much energy it is capable of producing—is highest among all types of power plants.

Finally, because they are abundant in the United States, geothermal resources offer a large source of secure energy to the nation's energy portfolio. Geothermal electricity production can help reduce the need for oil imports, reducing the trade deficit and adding jobs to the U.S. economy.

All these attributes make geothermal energy attractive for international and domestic markets. In the developing countries, demand for electric power is burgeoning—and nearly half these countries have geothermal resources. These markets represent promising export options for geothermal technologies and expertise. By one estimate, there are more than 80,000 MW-electric of potential for geothermal electrical power projects in developing countries using hydrothermal resources alone.

In addition, there are myriad opportunities in the United States to take advantage of direct-use applications. Eighteen district heating systems are now in operation, with the potential of many more. A district heating system is a community in which the buildings are heated geothermally. A recently updated resource inventory of 10 western states identified 271 communities located within 5 miles of a geothermal resource. Particularly well-suited industrial and commercial applications include greenhouses, fish farms, food dehydration, laundries, gold processing, milk pasteurizing, and swimming pools and spas.

The cost of generating power from geothermal resources has decreased about 25% over the past two decades. A power plant built today would probably require about $0.05 per kWh.

The goal of the geothermal industry and the U.S. Department of Energy is to achieve a geothermal energy life-cycle cost of electricity of $0.03 per kWh. Costs in this range will likely result in about 10,000 MW of new capacity installed by U.S. firms within the next decade.

Geothermal heat pumps are already boasting low operating and maintenance costs, and usually, low life-cycle costs. Life-cycle cost is the total cost of the equipment, as well as operating and maintenance costs, spread over the useful life of the equipment. Consumption of electricity is reduced 30%-60% compared to traditional heating and cooling systems, allowing a payback of system installation in 2-10 years.

For more information on geothermal energy, visit the U.S. Department of Energy's Geothermal Technologies Program. For additional information on geothermal resources in the western United States, visit DOE's GeoPowering the West.